The present invention relates to a vehicular display apparatus, and more particularly, to a vehicular display apparatus capable of displaying vehicle drive information with high visibility in an arbitrary one display form selected from a plurality of display forms.
In general, a vehicle is furnished with various meters which, facing the driver's seat, display information indicative of the vehicle drive conditions, including the operating states of various parts of the vehicle, so as to permit the driver to grasp the drive conditions. The meters in the vehicle are assorted depending on the vehicle model, and include a speedometer, water temperature indicator, oil-pressure gauge, tachometer, fuel gauge, some other instruments, and warning lamp, for example. Generally, several meters are arranged on the dashboard of the vehicle. Also available is a combination meter which comprises several kinds of meters intensively arranged in a case.
In general, moreover, the meters are illuminated so that they can be satisfactorily observed by the driver. There are various methods for illuminating the meters. For example, an instrument board for the meters may be illuminated by means of illuminating lamps which are arranged in front of the board, i.e., on the driver's-seat side with respect to the board. According to a transmissive illumination (back-lighting) method, on the other hand, a display section, which is composed of meter graduations, etc. formed on or through an instrument board, is illuminated by means of illuminating lamps which are arranged on the back of the board.
In the case of the conventional meter of the transmissive-illumination type, which uses a low-luminance light source such as an incandescent lamp, however, a satisfactory luminance cannot be obtained due to insufficiency of light quantity if a meter glass with a low light transmittance is used. Inevitably, therefore, it is necessary to use a meter glass with a relatively high light transmittance. Accordingly, the degree of contrast between the display section and the background is so low that the visibility of the conventional transmissive-illumination meter cannot be improved. When using an incandescent lamp which emits light containing red components in large quantities, a bluish cap or the like should be put on the lamp, in order to obtain white light. Even though this is done, it is difficult to obtain pure-white light.
With regard to the display form for a vehicular meter, it is known to make an analog display in which values are continuously indicated by rotary positions of a pointer on an instrument board, and a digital display in which digits are displayed. Analog display and digital display are different in properties related to visibility. In the case of a speedometer, for example, the analog display facilitates observation of speed variations, and therefore, is suited, e.g., for the indication of the speed of a vehicle running in an urban district or between hills or mountains where acceleration and deceleration are repeated frequently. On the other hand, the digital display facilitates an accurate reading of the speed, so that it is suited, e.g., for the indication of the speed of a vehicle running at constant speed on a freeway. Selection between the analog and digital displays also depends on the driver's taste.
Thus, in order to meet a wider variety of needs, there has recently been proposed a combination meter of the two-mode type which comprises, for example, analog and digital speedometers. According to the combination meter of this type, the display mode can be switched between the analog and digital display modes automatically in accordance with the running mode or by the driver's preference.
Typically, the conventional two-mode combination meter comprises a surface glass, which is formed by sticking a liquid crystal panel and a half-mirror together, and a liquid crystal panel drive unit for variably adjusting the light transmittance of the panel. For the analog display, the light transmittance of the liquid crystal panel is increased so that the surface glass becomes transparent, and the analog speedometer is illuminated indirectly by means of a cold-cathode discharge tube which is situated in front of and below the instrument board of the meter. Thus, the driver can visually perceive the analog speedometer as a real image through the surface glass. For the digital display, on the other hand, the light transmittance of the panel is lowered so that the surface glass becomes opaque, and a display on the digital speedometer, which is situated in front of and diagonally above the surface glass, is reflected toward the driver's by the surface glass. Thus, the driver can observe the meter display as a virtual image.
However, this conventional two-mode meter has many problems. For example, the surface glass should be selectively made transparent or opaque to ensure the mode switching between the analog display and digital display. Accordingly, the liquid crystal panel and the panel drive unit for changing the light transmittance of the panel are essential components, so that the meter entails high cost. When the temperature is so low that the light transmittance of the liquid crystal panel cannot be changed in a moment, moreover, the display mode cannot be instantly switched between the analog display and the digital display. Further, the use of the liquid crystal panel narrows the visual angle in the analog display mode. Since the instrument board is illuminated indirectly, moreover, the degree of contrast between the display information and the background in the analog display mode is so low that the visibility of the meter is lowered. The light transmittance of the liquid crystal panel must be increased in order to improve the visibility. In some cases, furthermore, the analog display and the digital display may be superposed on each other.